1. Field of the Invention
This invention relates to a method for reducing sheeting during polymerization of alpha-olefins and more particularly to a method for reducing sheeting during polymerization of polyethylene utilizing titanium based catalysts or vanadium based catalysts with alkyl aluminum cocatalysts.
2. Summary of the Prior Art
Conventional low density polyethylene has been historically polymerized in heavy walled autoclaves or tubular reactors at pressures as high as 50,000 psi and temperatures up to 300.degree. C. or higher. The molecular structure of high pressure, low density polyethylene (HP-LDPE) is high complex. The permutations in the arrangement of their simple building blocks are essentially infinite. HP-LDPE's are characterized by an intricate long chain branched molecular architecture. These long chain branches have a dramatic effect on the melt rheology of these resins. HP-LDPE's also possess a spectrum of short chain branches, generally 1 to 6 carbon atoms in length. These short chain branches disrupt crystal formation and depress resin density.
More recently, technology has been provided whereby low density polyethylene can be produced by fluidized bed techniques at low pressures and temperatures by copolymerizing ethylene with various alpha-olefins. These low pressure LDPE (LP-LDPE) resins generally possess little, if any, long chain branching and are sometimes referred to as linear LDPE resins. They are short chain branched with branch length and frequency controlled by the type and amount of comonomer used during polymerization.
As is well known to those skilled in the art, low pressure, high or low density polyethylenes can now be conventionally provided by a fluidized bed process utilizing several families of catalysts to produce a full range of low density and high density products. The appropriate selection of catalysts to be utilized depends in part upon the type of end product desired, i.e., high density, low density, extrusion grade, film grade resins and other criteria.
The various types of catalysts which may be used to produce polyethylenes in fluid bed reactors can generally be typed as follows: